I'm picking up rumours, whispers, that the chemical weapons were supplied to a tame rebel outfit by the Saudis and that the rebels either deployed them- with a view to landing Assad in the shit- or set them off by accident. I don't know the status of these reports (you'll find them at a site called Zero Hedge) but they're not implausible.
Peter Hitchens has a good line about the armed response that Cameron was deflected from and Obama has deferred. "The Good Samaritan," he writes, "Didn't have a gun".